The phenomenon of virtual diodes occuring at metallic junctions has been known for several decades. This characteristic of metallic junctions has been utilized in an effort to electronically differentiate metallic targets from nonmetallic surroundings since the mid 1960's with varying degrees of success. The detection range for a given power in the illuminating signal has been somewhat limited. The relatively low receiver signal levels make existing systems especially susceptable to jamming or other interfering signals. In prior systems an attempt was made to use high metallic junctions in the target to saturate or become inoperative. To overcome these shortcomings, in some system environments where the target was already of such character that it could cooperate with the balance of the system, higher efficiency non-linear elements were purposely located on the target. This is seldom practical, however, in non-cooperative target environments.